Laika was a stray from the streets of Moscow, chosen for her calm temper. Sputnik 2 was built in about four weeks to mark the 40th anniversary of the October Revolution, and there was no way to bring it back to Earth.
For years Soviet officials said she had lived for several days. In 2002 it was revealed that she died of overheating a few hours after launch. Her flight still showed that a living creature could survive launch and weightlessness.
Source: Wikipedia